Would you hire a webmaster when you find out he doesn't have his own website?
Would you hire an SEO expert when you find out you can't find him on the web?
When dealing with staffers, partners, clients, etc., I look at their presence: do they have websites? Are they findable? Are they on LinkedIn and Facebook?
It tells a great deal about their awareness and sophistication.
Sure, you can lie. But what happens when a potential employer, client (or even a potential girlfriend) realizes that you lied? That's even worse than not being on the web at all (i.e., a luser).
Look at LinkedIn. It's become the industry standard for professionals. If you're a manager, director, or higher, you'd better be in LinkedIn. And it has to be accurate, just like a resume.
The issue of fake profiles is minor; I've read that only about 3% of people lie about their income, education, etc. in surveys.
This is another way in which FB and Li (LinkedIn) are changing the web. Google wasn't able to do this. It could only index the web, but it didn't have much impact on the content. FB and Li are adding people's identities (and social persona) to the web. When you go to a page, you see your friends also know that page. The web becomes personalized.